Digital transmission and switching systems traditionally have decoded Nyquist rate PCM signals directly and then used analog filters to smooth the resulting pulse amplitude modulation. There is now increasing incentive to replace the analog filters with digital filters in order to provide greater dynamic range in both encoder and decoder, and thus provide range for additional management functions such as gain control, hybrid balancing, echo suppression and conferencing.
One digital decoding procedure that has been proposed raises the sample rate of the digital input signal with digital circuits which generate extra sample values. Thereafter, demodulation at the high sample rate eliminates the need for analog filters at the output, and permits the use of only a few uniformly spaced analog levels to represent output amplitudes.
While various techniques for increasing the frequency of the input samples have been proposed, it has been generally found most efficient to raise the sample rate in several stages, rather than all at once. One technique for raising the frequency uses simple repeating, whereby input words enter a register from which they are each read several times in succession. This register output is then smoothed by a low pass filter. Alternatively, the word rate can also be increased by simple linear interpolation, wherein several new sample values are inserted between successive input samples, and such interpolation is the subject of the present invention. If performed efficiently, interpolation smooths the signal, attenuating all images of the baseband except those adjacent to the desired output sampling frequency.
At present, interpolation is often performed by computing the difference between successive input samples, dividing the difference by the desired number of output samples to be generated per input interval, and incrementing an accumulator which stores the previous interpolator output. Interpolators of this type are usually extremely sensitive to offsets which may occur during the processing, since the error continues indefinitely once it has been inserted in the accumulator loop. In addition, the accumulator must be initialized each time it is turned on in order to eliminate any residual errors, thereby adding to the complexity of the required circuitry.
In view of the foregoing, it is the broad object of the present invention to provide an improved interpolator in which offsets are not indefinitely accumulated and in which initialization is not difficult. Other objects are to provide an interpolator which is both efficient and easily fabricated using integrated circuit technology.